1. Technical Field
The subject invention relates to the synthesis volatile organometallic compounds having the formula EQU MR.sub.n
where M is a metal from groups IIB, IIIB or IVB of the Periodic Table, each R is independently selected from alkyl and aryl and combinations thereof; and n is an integer determined by the valence of M.
2. Background Art
Compounds between elements of group III-A and of group V-A of the Periodic Table are of economic value as semiconductor materials in electronic and optoelectronic applications. To be suitable for use in electronic devices, the III-V materials must be prepared under very stringent conditions of chemical purity. The current state of the art is such that the requisite conditions to deposit thin films of III-V compounds can be attained only by a few sophisticated deposition methods, such as OMVPE (organometallic vapor phase epitaxy) or MOCVD (metal organic chemical vapor deposition) and MBE (molecular beam epitaxy). P. Zanella, et al., Chemistry of Materials 3, 225-242, 225 (1991).
In such approaches, a common method of introducing the Group II, III, IV and V compounds is as a volatile metal alkyl compound (e.g., trimethylindium). P. Zanella, et al., at page 226. It is generally accepted that the purity level of these precursor alkyls currently limits the obtainable purity of the resultant epitaxial layer of III-V compound, which in turn determines the technological. usefulness of the resultant device. P. Zanella, et al., at page 226. Accordingly, techniques which limit the levels of impurities in the metal alkyl compounds are of great value in improving the quality of the III-V semiconductor.
Because the metal alkyl compounds are typically transferred in the vapor phase, the most detrimental impurities are those which are volatile. Both the method of synthesis and the method of purification determine what volatile impurities will be present.
Consider the illustrative case of indium, a group IIIB metal. There are several known methods of synthesis of trialkyl indium compounds:
1. In+1.5 HgR.sub.2 .fwdarw.R.sub.3 In+1.5 Hg L. M. Dennis, et al., J. Am. Chem. Soc. 56, 1047 (1934); P. Krommes, et al., J. Inorg. Nucl. Chem. Letters 9, 587 (1973). This method can leave behind volatile and toxic mercury and mercury compounds. PA0 2. InCl.sub.3 +3RLi.fwdarw.R.sub.3 In+3 LiCl H. C. Clark, et al., J. Organometal. Chem. 8, 427 (1967). The alkyllithium reagent reacts with trace oxygen and water, and must be standardized immediately before use. After exposure to air or water, the alkyllithium reagent can be contaminated with R.sub.2 and R-OH species, which must be removed from the R.sub.3 In product. PA0 3. InCl.sub.3 +3 RMgCl.fwdarw.R.sub.3 In+3 MgCl.sub.2 V. F. Runge, et al., Z. Anorg. Allg. Chem. 267, 39 (1951); D. F. Foster, et al., J. Chem. Soc., Dalton Trans., 7 (1988). The Grignard reagent RMgCl is pyrophoric. PA0 4. In+3.5 Mg+5 RBr.fwdarw. R.sub.3 In+1.5 MgBr.sub.2 +2RMgBr V. E. Todt, et al., Z. Anorg. Allg. Chem. 321, 120 (1963). The reactions are slow and reactive by-products are formed. PA0 5. In+RMgCl+ether+electrolysis.fwdarw. R.sub.3 In J. B. Mullin, et al., U.S. Pat. No. 4,599,150 (1986). The reagent RMgCl can react with air and water. There is a low yield of R.sub.3 In product. PA0 6. InCl.sub.3 +3 R.sub.3 Al+3 KCl.fwdarw.R.sub.3 IN+KAlR.sub.2 Cl J. J. Eisch, J. Am. Chem. Soc. 84, 3605 (1962); A. C. Jones, et al., J. Cryst. Growth 77, 47 (1986); A. H. Moore, et al., J. Cryst. Growth 77, 19 (1986); R. B. Hallock, et al., U. S. Pat. No. 4,847,399 (1989). The reagent R.sub.3 Al can react with air and water. There is a low yield of product.
Methods 1-3 and 5-6 all use reactive, liquid-phase reagents which react with air and water to create secondary impurities which must be removed from the R.sub.3 In product. The liquid-phase reagents must be standardized before use. Method 4 creates a reactive, liquid-phase product which reacts with air and water. Thus, all previous methods involve working with a reactive, liquid-phase reagent which creates difficulties during synthesis and additional complexity in the purification step.
How effectively the metal alkyl compound can be purified determines the impurity level and value of the final semiconductor product. To remove volatile impurities from volatile metal alkyl compounds such as trialkyl indium, the art teaches a purification process comprising:
a) reaction of the volatile trialkyl metal compound to form a product of lower volatility. The art teaches complex formation of the trialkyl metal compound with a Group VB-containing Lewis base. D.C. Bradely, et al., U.S. Pat. No. Re 33,292 (1990); and D. F. Foster, et al., J. Chem. Soc., Dalton Trans., 7 (1988). The art also teaches adduct formation in the presence of excess R.sub.x M, where R=alkyl group, M=a group I-A or II-A metal. R. B. Hallock, et al., U.S. Pat. No. 4,847,399 (1989); PA1 b) pumping off the volatile impurities. Of course, any impurities which became involatile because of the chemistry of complex or adduct formation will not be removed; and PA1 c) regeneration of the volatile trialkyl metal. PA1 a) placing a sublimable compound containing volatile impurities in a closed system at a temperature at which the sublimable compound is a solid; PA1 b) establishing a vacuum in the system by pumping away any gases which may be present, either by employing conventional freeze-pump-thaw methods or by pumping until the pressure drops to a nearly steady state; PA1 c) isolating the system after establishing a vacuum as described above; PA1 d) maintaining one portion of the system which is away from the sublimable compound at a lower temperature than that of the sublimable compound to cause the compound to crystallize at such portion of the system at a rate which excludes impurities from the crystallizing compound; PA1 e) removing any volatile impurities liberated during the recrystallization by evacuating the system until the pressure reaches a nearly steady state; and PA1 f) repeating steps a) to e) until a desired purity is achieved.
The art thus teaches a three step approach to purification involving conversion of the trialkyl indium to nonvolatile species, pumping of impurities, and regeneration of volatile species.